1. Field of the Invention
This invention relates to a method for determining the source region of a volcanic tremor caused by subterranean volcanic magma motion by measuring the volcanic tremor at at least four observation points.
2. Prior Art Statement
Similarly to the technique used in ordinary determination of the hypocenter of local earthquakes, the conventional technique for determining the source region of a volcanic tremor uses the difference in arrival time of P waves (longitudinal waves and compressional waves) and S waves (transverse waves and distortional waves) measured at at least four observation points or otherwise uses the times at which initial motion commences.
When the volcanic tremors are from a source region near the Earth's surface, however, it is frequently found that the P waves and S waves are not clearly distinguishable and, moreover, that the initial motion times cannot be accurately ascertained. Precise calculation of the tremor source region has therefore been difficult using the conventional hypocenter determination method.
Since this has made it impossible to obtain accurate knowledge regarding the source of volcanic tremors caused by magma motion even when the magma is near the surface (ordinarily within 20 km of the surface), it has been difficult to use volcanic tremor measurements for predicting volcanic eruptions accurately.
In U.S. Pat. No. 4,961,143 the inventors proposed a method for predicting volcanic eruptions comprising the steps of simultaneously receiving electromagnetic waves produced by magma via antennas installed at four points on land and/or the seabed, cross-correlating the received signals to detect time lags among the signals, calculating the size, magnitude and shape of the magma, which generates long-wave radiation, from the time lags, and predicting the likelihood of a volcanic eruption from time-course changes in the calculated size, magnitude and shape of the magma.
With this method, however, the fact that electromagnetic waves are attenuated by the Earth's crust makes difficult to conduct the required measurements when the magma is at a depth of 10 km or more from the surface. Another problem with the method is that its dependence on the reception of electromagnetic waves makes it necessary to use expensive observation equipment.